1. Field of the Invention
The present invention relates to a method for testing pulp samples used to make paper products, and, more particularly, to a test method for determining the amount of contaminants that would create bleed-throughs in a product manufactured from the pulp.
2. Description of the Related Art
The use of old corrugated container (OCC) for making new containerboard increased rapidly in the 1980s and early 1990s. According to American Forest and Paper Association, the recycling rate of OCC, defined as the ratio of the tonnage of OCC grade fiber used at paper and paperboard mills to that of total containerboard supply, increased from 39.5% in 1987 to 63.4% in 1996. Since then, the recycling rate has stabilized at about 63.5%.
At such high recycling rates, removal of contaminants from OCC becomes crucial to assure quality standards of the products made from OCC fiber. Standard processes to recycle OCC consist of a pulper, detrasher, high-density cleaner, coarse screen, forward cleaner, fine screen, reverse or through-flow cleaner and a thickener. Although the majority of large contaminants can be removed by such a processing sequence, many fine contaminant particles remain in the processed pulp. These fine particles may either be brought in by the contaminated OCC, or generated by the fragmentation of larger particles during the treatment process. Tests on samples of fine screen accepts indicate that the size of the fine contaminants ranges from 80 xcexcm to 750 xcexcm in equivalent diameter, with an average of about 170 xcexcm. Therefore, the majority of the fine contaminants can pass easily through a 0.20 mm (200 xcexcm) slotted screen basket, which is the type of basket used in most OCC recycle mill fine screen systems. Many large particles also are able to pass through the screen baskets, due to the shape or flexibility of the particle. Reverse and through-flow cleaners have also proven to be ineffective in removing these contaminants, since the specific density of xe2x80x9cbleed-throughsxe2x80x9d are close to 1, and the size of the contaminant is too small to create sufficient drag forces to differentiate the contaminant from pulp fibers.
Failure in removing the fine contaminants, together with the increasing usage of OCC, has caused a growing deterioration in the qualities of recycled pulp. One of the greatest concerns regarding pulp quality is the formation of objectionable xe2x80x9cbleed-throughsxe2x80x9d in dried paperboard. xe2x80x9cBleed-throughsxe2x80x9d are formed from the melting of fine contaminants. At the elevated temperature (approximately 350xc2x0 F.) of a papermaking machine dryer section, many of the fine contaminants will melt and migrate into voids of the fiber web. At the spot occupied by a melted contaminant, a dark bleed-through is formed, which gives the finished paperboard an objectionable appearance. The concentration of xe2x80x9cbleed-throughsxe2x80x9d in paper sheets, tested with an image analyzer, can reach as high as 50,000 ppm, that is, 5% of the total sheet area is covered with dark xe2x80x9cbleed-throughsxe2x80x9d. The xe2x80x9cbleed-throughsxe2x80x9d also can cause problems in converting operations, such as brown tissue embossing and the making of gypsum-board.
The worst situation occurs in mills using 100% recycled fiber. In these mills, each time the OCC is recycled, more contaminants are brought in to the fiber stream. The contaminants accumulate with each recycling of the fiber, and eventually the pulp becomes unsuitable for making new paper products because of the high concentration of contaminants.
Methods have been proposed for more efficient removal of small contaminants that form xe2x80x9cbleed-throughsxe2x80x9d in the final products. One such method is disclosed in our co-pending application, U.S. Ser. No. 09/800,031 filed Mar. 6, 2001, now U.S. Pat. No. 6,425,982, entitled, xe2x80x9cSystem and Method For Removing Bleed-Throughs From Old Corrugated Container Fiber Pulp.xe2x80x9d
A problem in processing OCC pulp by any process is determining the level of contamination in the pulp by contaminant particles that would result in xe2x80x9cbleed-throughsxe2x80x9d in a final product. It is advantageous in processing such a pulp to know the level of contamination in the pulp, and to evaluate the effectiveness of the process for removing the contaminants. It is also desirable to have a reliable test method for testing the contaminant level at the inlet and the outlet of equipment in the removal process, so that the efficiency of the equipment, for the pulp being processed, can be evaluated. Since many of the contaminants are small, and often not readily apparent in the pulp until after manufacture of the product, known testing procedures are often ineffective in determining the level of contamination in the pulp. Often the level of contamination is low, and the sources of the contamination are complex and ever changing. The particles are small, and difficult to remove from the fibers.
What is needed in the art is a test method for determining in OCC pulp the level of contamination by particles that would form xe2x80x9cbleed-throughsxe2x80x9d in the final product.
The present invention provides a test method for determining the level of xe2x80x9cbleed throughsxe2x80x9d contamination in OCC fiber pulp, including forming a sheet of the pulp to create the xe2x80x9cbleed-throughsxe2x80x9d, dying the pulp fibers to enhance the contrast between the xe2x80x9cbleed-throughsxe2x80x9d and the fiber, and analyzing the sample with an image analyzer.
The invention comprises, in one form thereof, a test method for measuring the bleedthrough contaminant content of a fiber pulp. The method steps include obtaining a sample of the pulp; forming a paper web from the sample of the pulp; drying the web to form a sheet; melting the bleed-through contaminants in the sheet; enhancing the contrast between areas of the sheet having bleed-through contaminants and areas of the sheet not having bleed-through contaminants; and analyzing the contrasted sheet for ascertaining the concentration of bleed-throughs in the sheet.
The invention comprises, in another form thereof, a test method for analyzing an old corrugated container fiber pulp, to evaluate the content therein of contaminants creating bleed-throughs in a paper product formed from the pulp. The test method includes steps of obtaining a sample of the OCC pulp; forming a handsheet from the sample; heating the handsheet to create bleed-throughs from the contaminants in the handsheet; enhancing contrast between regions of the handsheet having bleed-through contaminants and regions of the handsheet not having bleed-through contaminants; and analyzing the enhanced handsheet to determine the concentration of bleed-throughs in the sheet.
The invention comprises, in a further from thereof, a test method for analyzing an old corrugated container fiber pulp, to evaluate the content therein of contaminants creating bleed-throughs in a paper product formed from the pulp. The test method includes steps of obtaining a sample of the pulp; forming a paper sheet from the sample, including creating bleed-throughs from the contaminants contained in the sheet, enhancing contrast between regions of the sheet having bleed-throughs and regions of the sheet not having bleed-throughs; and analyzing the enhanced sheet to determine the determine the concentration of bleed-throughs in the sheet.
An advantage of the present invention is that the amount of fine contaminants in a pulp sample, which cause bleed-through flaws in a fiber web, is determined accurately.
Another advantage is that the test method simulates a paper forming process, to more accurately evaluate the level of contamination by contaminants that would actually form xe2x80x9cbleed-throughsxe2x80x9d.
A further advantage is that the test method of the present invention uses standard pulp and paper testing procedures, and equipment commonly found in pulp and paper testing laboratories, and does not-require investment in expensive equipment not useful for other testing purposes.